Add Client Event Observation

[toddbaert]

Add event observation methods to the Client interface for parity with OpenFeatureAPI.observe().

Specification References:

• 5.2.1: The client MUST provide a function for associating handler functions with a particular provider event type.

Implementation Notes:

• Add Client.observe() -> AnyPublisher<ProviderEvent?, Never> method
• Currently only OpenFeatureAPI.shared.observe() exists
• Client-level observation should filter to events from the client's associated provider